Parathyroid hormone (PTH) is an 84 amino acid polypeptide essential for the maintenance of calcium and phosphate homeostasis. Fragments of PTH, as well as the intact hormone, circulate in patients with hyperparathyroidism. These PTH fragments have been identified primarily by immunochemical techniques, and little is known of their biologic role. To address this question, we have developed a highly sensitive PTH bioassay based upon stimulation of renal cortical adenylate cyclase activity in the presence of the amplificatory guanyl nucleotide analog, 5'-guanylimidodiphosphate. This PTH bioassay will be used to determine whether hyperfunctioning human parathyroid glands secrete biologically active PTH fragments and whether such fragments differ in primary versus secondary hyperparathyroidism; to determine the quantity and origin of circulating, biologically active PTH fragments; to relate the biologic activity of these fragments to their immunochemical activity; and to develop and validate a procedure for the routine measurement of PTH bioactivity in peripheral serum. PTH fragments will be isolated by gel filtration chromatography of parathyroid venous effluent sera and corresponding peripheral sera from patients with primary and secondary hyperparathyroidism. Biologic assay and radioimmunoassay (region-specific PTH antisera) of these fragments will provide evidence of their quantity, origin, molecular size, and immunochemical properties. Direct biologic assay of PTH in peripheral serum will be validated by relating bioassay values with other indices of parathyroid function (i.e., serum calcium and nephrogenous cyclic AMP excretion) in patients with primary hyperparathyroidism. It is anticipated that this biologic assay will ultimately provide a useful adjunct to radioimmunoassay for the measurement of PTH in the serum of patients with diseases of parathyroid function.